Sister Cities International (SCI) is a non-governmental organization (NGO) with the goal of facilitating partnerships between communities within the United States and other countries by establishing sister cities. Sister cities are agreements of mutual support formally recognized by the civic leaders of those cities. A total of 1,800 cities, states, and counties have partnered in 138 countries.

A sister city or a twin town relationship is a form of legal or social agreement between two geographically and politically distinct localities for the purpose of promoting cultural and commercial ties.